1. Field of the Invention
The present invention relates to an image sensor containing plural photodiodes.
2. Related Background of the Invention
An image sensor containing plural photodiodes is used for a spectroscopic device, for example. In the spectroscopic device, light in some wavelength band which is output from a light source is made incident to a sample, light which is a part of the incident light and transmitted through the sample is spectroscopically separated by a spectroscope, and then the spectroscopically separated light beams having respective wavelengths are detected by the image sensor. In such a spectroscopic device, it is common that the intensity spectrum I(λ) of light output from the light source and the detecting sensitivity spectrum D(λ) of each photodiode contained in the image sensor are not flat in wavelength bands used for spectroscopic measurements.
Furthermore, the photodiode generates charges whose amount corresponds to the amount of light received, and accumulates the thus generated charges into a junction capacitance portion. When the junction capacitance portion is electrically connected to another circuit, the charges which have been accumulated in the junction capacitance portion until then are transmitted to the circuit concerned. However, the amount of charges to be accumulated in the junction capacitance portion of the photodiode is limited. When the amount of light received is small, the accumulated charge amount is proportional to the amount of light received, however, when the amount of light received is large, the accumulated charge amount is saturated.
As described above, it is required to set the charge accumulation time in the spectroscopic device so that the accumulated charge amount in the junction capacitance portion of the photodiode is not saturated at a wavelength λ at which the product (I(λ)·D(λ)) with the intensity spectrum I(λ) of light output from the light source and the detecting sensitivity spectrum D(λ) of the photodiode has a peak. An image sensor contained in a spectroscopic device disclosed in Japanese Patent No. 3,572,681 is designed so that the charge accumulation time is set to be as long as possible to the extent that the accumulated charge amount is not saturated at a wavelength λ at which the product (I(λ)·D(λ)) has a peak, whereby a spectroscopic measurement having high sensitivity can be performed.